Satellite telephone systems are emerging as a new and important global business. These systems utilize many individual circuits routed through one satellite or a constellation of many satellites to provide communications for terrestrial terminals. One significant advantage of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth without requiring the construction of many small terrestrial cells.
Since a portion of the frequency spectrum has become available for these services, several proposals have been advanced for utilizing this spectrum. One proposal would use Time Division Multiple Access (TDMA), and several others would use Code Division Multiple Access (CDMA). An important feature of the CDMA systems is the ability to share the frequencies by co-frequency operation.
Traditionally, Frequency Division Multiple Access (FDMA) and TDMA systems have been "Band Split" in order to share a band of frequencies. For example, if a frequency band of width W is shared equally by two systems the resultant bandwidth for each system is W/2. However, two CDMA operators that would share the same bandwidth W would see considerably different results. The two CDMA operators would share the frequencies by operating co-frequency and share "interference" with each other. That is to say, the CDMA waveforms transmitted by each system would experience noise from the other system, and this noise would be present in their respective receivers when trying to recover the waveform that had been transmitted. Thus, one CDMA system will experience interference from another system. The two CDMA systems will add their respective capacities, less the amount of interference caused by each other. In effect, the sum or total capacity of the two CDMA systems can be greater than the whole.
In general, well-coordinated CDMA systems should be able to operate co-frequency in the presence of each other with about a 25% to 35% degradation in each system. Therefore, in this example, system One with a capacity of C when operating in the presence of System 2 with a capacity of D, once coordination has been effected, will have, by example, a capacity of C-25% and System Two will have a capacity of D-25%. If the systems were of the same efficiency and capacity then the resultant capacity of the spectrum used would be 1.5.times.C or 1.5.times.D.
In this discussion it is assumed that the two systems have "coordinated" with each other and exchanged technical data which allows co-frequency operation. Essential to this, for CDMA systems, is the concept of "coordinating" the Power Flux Density (PFD), at the ground, which is transmitted from each satellite. In general, the process contemplates each system operating with a maximum PFD, which is typically set by external standards such as the International Telecommunications Union (ITU) in order to prevent satellite interference into terrestrial systems. Of course, the two systems will typically not always be operating at the maximum PFD.